JP2002126857A - Magnetic field generator for continuous casting of steel and continuous casting method of steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable continuous casting at a high speed by enabling the oscillation (mold oscillation) at the high vibration number. SOLUTION: In the magnetic field generating device for continuous casting, a coil case fixed with an annular hollow super conducting coil is supported by horizontal cables becoming a double cylinder structure from its back face, and by supporting with at least three pulling cables from its up and down sides, the coil case is engaged at the prescribed position inside the vacuum vessel.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic field generator for continuous casting of steel and a continuous casting method of steel using the same, and more particularly to a method for efficiently producing high quality cast slabs.

[0002]

2. Description of the Related Art In continuous casting, a molten metal (molten metal), for example, molten steel is generally injected from a tundish through an immersion nozzle into a mold. However, since the molten steel discharged from the discharge port of the immersion nozzle has a large discharge flow rate, inclusions such as alumina, bubbles, and the like may be caught in the steel, which contribute to product defects. Was. Such defects caused by inclusions such as inclusions and bubbles have a strong correlation with the discharge flow rate of molten steel, and as the discharge flow rate increases, product defects also increase. At present, the discharge rate of molten steel has reached 4 to 6 ton / min, but it is difficult to increase the discharge rate further unless drastic improvement of the continuous casting method is performed.

Further, at the time of molten steel injection, the discharge flow collides with the short side of the mold. When this impinging flow is regenerated into an upward flow along the short side wall of the mold, this upward flow causes the molten metal surface to be disturbed. As a result, mold flux and the like added to the molten metal surface are entrained in the steel, which is also a product. This was a cause of quality deterioration.

In the past, such problems have been addressed by devising the shape and angle of the nozzle discharge port to decelerate the molten steel discharge flow. However, there is a tendency to increase throughput (amount of casting) for higher efficiency and higher economic efficiency. For this purpose, the conventional method is not sufficient, and the development of a new casting method has been desired. .

[0005] As a prior art relating to this point, a technique of applying a static magnetic field (static magnetic field by a normal magnet) to molten steel in a mold to apply a brake to the molten steel discharge amount to prevent inclusions from entering the inside of the steel. (For example, Japanese Patent Laid-Open No.
52549, JP-A-8-19841, JP-A-57-17
356, JP-A-2-284750, etc.). However, in these prior arts, although the direction of the molten steel jet can be changed, the energy of the jet cannot be dispersed to form a uniform flow. Further, the molten steel may escape to a region where there is no static magnetic field. Furthermore, throughput 4
Although the quality improvement effect can be seen up to about 5 ton / min, there is a problem that the effect cannot be expected at a throughput exceeding this.

As an attempt to overcome such a problem, Japanese Patent Application Laid-Open Nos. 8-90176 and 8-229648 have proposed a continuous casting method of steel using a superconducting magnet. In addition, it is possible to advantageously solve the above problem at the time of high-speed casting operation.

[0007] As is well known, in continuous casting,
The mold is vibrated to improve lubrication between the mold and the slab.
Therefore, in order to increase the casting speed, it is necessary to increase one or both of the stroke and the frequency of the mold vibration (called oscillation). However, if the stroke is too large, there is a possibility that entrapment of the solid powder in the molten steel meniscus portion in the mold or blockage of the powder flow path by the slag rim may be caused, so that the stroke is usually set to 10 mm or less. Therefore, it is necessary to increase the frequency of the mold vibration from the viewpoint of increasing the casting speed. Also,
From the viewpoint of the slab surface properties, increasing the mold frequency has the advantage that the depth of the on-line mark can be reduced.

However, in the casting method using a superconducting magnet, the magnetic field is extremely large as compared with the conventional normal magnet, and when the mold is vibrated in this magnetic field, a large reaction force is generated in the anti-vibration direction. I do. This reaction force is proportional to the strength of the magnetic field, the speed of the object, and the electric conductivity according to Faraday's law. On the other hand, as described above, it is desirable to perform casting at a higher speed for quality and casting stability, and accordingly, it is necessary to increase the vibration frequency of the mold to some extent. The power is extremely large. For example, when the magnetic field strength is 1 T and the mold vibration frequency is 120 cpm, this reaction force reaches 1 ton or more. For this reason, a large load is applied to the actuator for exciting the mold, and this load is further increased particularly when an attempt is made to increase the frequency or to increase the stroke.

Further, according to Newton's third law (law of action / reaction), the above-mentioned reaction force is applied not only to the mold but also to the superconducting magnet which is a magnetic field source. Therefore, an excessive force acts on the coil of the superconducting magnet and the support member supporting the coil. For this reason, even if high-speed casting and high-throughput production are performed using superconducting magnets while maintaining high quality, such high-speed casting and high-throughput production cannot be performed in practice. is the current situation.

[0010]

DISCLOSURE OF THE INVENTION The present invention has been developed in view of the above-mentioned situation, and is a steel which enables an operation (mold vibration) at a high frequency and which enables continuous casting at a high speed. It is an object of the present invention to propose a magnetic field generator for continuous casting, together with a continuous casting method for steel using the same.

[0011]

As in the prior art, in the casting using a static magnetic field with a normal magnet, the magnetic field is not so strong, and the casting speed and the throughput are small, so that the mold oscillation frequency is not high. Therefore, there was no problem with the mold reaction force. However, when performing a high-speed casting by applying a static magnetic field using a superconducting magnet, the operation of the mold is hindered because the mold reaction force increases as described above.

FIG. 1 is an explanatory view of the principle of generating a mold reaction force. In the drawing, reference numeral 1 denotes a mold, 2 denotes a superconducting magnet coil, and the magnetic field 3 generated by the superconducting magnet coil 2 vibrates the mold 1 under a state where the magnetic field 3 penetrates the long side wall of the mold 1 vertically. Then, the induced current 4 flows inside the mold. The interaction between the induced current 4 and the magnetic field 3 generates a mold reaction force 5. Thus, when a copper plate is vibrated in a magnetic field, a very large reaction force acts on the copper plate.

In order to solve this problem, the present inventors have previously used a plate having a low electric conductivity as a plate constituting a mold and using a low electric conductive material or an insulating material as a part of the plate. A method for reducing the reaction force of the mold during vibration of the mold was proposed (Japanese Patent Application No. 11-36748). According to this method, the mold reaction force can be reduced, and the load on the drive system of the oscillation can be reduced.

However, even in this method, there still remains a problem that the support portion of the superconducting magnet coil is fatigued by the influence of the reaction force of the mold. In this regard, if the superconducting magnet is vibrated in synchronization with the mold, there is no relative movement with the mold, so that there is no reaction force to the coil in the superconducting magnet. Therefore, the inventors conducted intensive studies on a magnetic field generator having a structure suitable for applying a magnetic field while oscillating the superconducting magnet in synchronization with the mold, and completed the present invention after trial and error. It has been reached.

That is, the gist of the present invention is as follows. 1. A magnetic field generator that generates a static magnetic field in a direction opposite to the long side wall of the continuous casting mold, an annular air-core superconducting coil, and a coil case for fixing the coil,
A plurality of cords for locking the coil case at predetermined positions in the vacuum vessel, a refrigerator connected to the superconducting coil via a metal coupling body, and keeping the coil at a very low temperature by conduction cooling; And a vacuum device for controlling the pressure in the vacuum vessel. The coil case is supported by a horizontal cable having a double cylindrical structure from the back thereof, and supported by at least three pulling cables from above and below the coil case. A magnetic field generator for continuous casting of steel.

2. 1. The magnetic field generator for continuous casting of steel according to 1, wherein the coil case is supported from above and below by at least three pull cords on the back side of the coil case.

3. The magnetic field generator for continuous casting of steel according to 1 or 2, wherein a heat shield for blocking radiant heat from the slab is provided outside the magnetic field generator.

4. In the above 1, 2, or 3, continuous NbTi wire having a wire diameter of 5 μm or less is wound in a race track shape, and after winding, the coil is integrated by vacuum impregnation with epoxy resin. Magnetic field generator for casting.

5. When performing continuous casting of steel using a continuous casting mold provided with the magnetic field generating device according to any one of the above 1 to 4, while synchronously oscillating the mold and the magnetic field generating device, generated from the magnetic field generating device. A continuous casting method for steel, wherein the flow of molten steel in a mold is controlled using a strong static magnetic field.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. In the present invention, an object is to apply a strong magnetic field to a mold, and a superconducting magnet is used for that purpose. As such a superconducting magnet, an NbTi superconducting wire is wound by a solenoid winding in a race track shape having a width exceeding the mold width so that the required number of turns is uniform in the mold width direction. It is preferable to use a coil that is integrated by vacuum impregnation. Next, the coil having the shape of a race track is stored in a cryostat (cryogenic vessel) to perform cooling for bringing the superconducting state. Then, two or four such superconducting coils are connected in series, both ends of which are connected to a current introduction terminal of a cryostat, and excitation is performed using an excitation power supply attached to the superconducting magnet.

As described above, the superconducting coil of the present invention is an annular air-core coil, and does not have an iron core unlike a normal-conduction electromagnetic coil. One reason for this is to reduce the total weight of the coil as described in Japanese Patent Application Laid-Open No. Hei 8-229651, but in the strong magnetic field generated by the superconducting coil, the strength of the magnetic field in the iron core is saturated. Because it has no effect.

The inventors have studied a magnet structure capable of appropriately vibrating a superconducting magnet in synchronization with a mold. The problem here is how to prevent the vibration of the coil. Since the superconducting coil has no iron core and is air core, it is usually supported by a plurality of support parts. That is, conventionally, as shown in FIG. 2, the structure was simply suspended by the support member 6. However, in this structure, when the superconducting coil is vibrated in synchronization with the mold,
At the time of upward movement, it does not stop at a predetermined position and moves upward, and at the time of downward movement, the support material 6 is overloaded, and there is a concern that the support material 6 may be broken.

Therefore, the present invention has the following structure in order to realize appropriate vibration of the superconducting magnet synchronized with the mold. First, the superconducting magnet coil is fixed with a non-magnetic coil case (preferably, a stainless steel (SUS 304) coil case). This is to prevent the coil from being deformed by vibration. Also, when two sets of coils are used to apply two upper and lower magnetic fields, it is necessary to suppress the repulsive force acting between the two upper and lower coils. Then, as shown in FIGS. 3A to 3C, the coil case 7 is supported by the superconducting magnet coil 2 together with the horizontal cords 8 and the pull cords 9 so that the vacuum case of the coil case 7 is formed. Variations in the horizontal and vertical planes from a predetermined position within 10 are prevented even under vibration conditions.

That is, the horizontal cable 8 has a double cylindrical structure composed of an inner pipe made of carbon FRP and an outer pipe made of glass FRP that surrounds the inner pipe.
On the other hand, since the compression force can be supported by the glass FRP outer tube, the coil case 7 is supported from the back by the horizontal cable 8 having such a double cylindrical structure, so that the inside of the vacuum container 10 of the coil case 7 can be improved. In this case, the fluctuation in the horizontal direction from a predetermined position due to the electromagnetic force mainly acting on the opposing coils can be effectively prevented.

On the other hand, the fluctuation in the vertical plane can be prevented by pulling and supporting from at least three (four in this example) pulling ropes 9 from above and below. FIG. 3 shows a case where the coil case 7 is supported by the four pulling ropes 9, but as shown in FIG. 4, if there are at least three pulling ropes 9,
Fluctuation in a vertical plane from a predetermined position of the coil case 7 in the vacuum vessel 10 can be effectively prevented. In FIG. 3 (b), the pull cord 9 is shown to be inclined slightly to make it easier to see, but it is actually installed almost vertically.

Here, the material of the horizontal cord and the pull cord is not particularly limited, but the above-mentioned FRP (carbon fiber or glass thread fiber reinforced plastic) is used in view of strength and heat conduction. Desirable. That is, heat conduction is small, and the strength of each wire is about 1.0 to 1.5 × 10 ⁴ N.

In FIG. 3, reference numeral 11 denotes a refrigerator. The refrigerator 11 is connected to the superconducting coil 2 via a metal coupling body 12 made of Al or Cu. It can be kept at cryogenic temperatures. In addition,
As the refrigerator 11, a two-stage refrigerator is suitable. Here, the two-stage refrigerator has a two-stage structure in which the heat absorbing portion of the refrigerator main body has a one-stage stage (80 K level heat absorbing portion) and a two-stage stage (4 K level heat absorbing portion). The first stage is used for cooling the radiation shield plate, and the second stage is used for cooling the superconducting coil.

By using such a conduction cooling type refrigerator, liquid helium which has been conventionally used in order to make the superconducting coil 2 operable at 4 to 5K becomes unnecessary, and thus the entire apparatus is downsized. As a result, it was possible to install the mold. Numeral 13 is a multilayer heat insulating sheet for blocking the transfer of heat between the inside of the vacuum vessel 10 and the outside. It is preferable that the heat insulating sheet 13 is formed by stacking several tens of sheets of Mylar sheet on which Al is vapor-deposited, so that the inside of the vacuum vessel can be more appropriately kept at a low temperature.

Thus, even when the frequency of the mold is increased, the continuous casting operation can be performed at a high speed without adversely affecting the superconducting coil.

By the way, when continuous casting using a superconducting magnet was performed for a long period of time, a case where the superconducting state of the superconducting coil disappeared occurred. In other words, when applying the superconducting magnet to the continuous casting method, it is designed so that it can be sufficiently applied to the actual continuous casting machine, and after conducting the actual machine test, it shows that it can be used Despite the fact that it was installed on a real machine, it nevertheless occurred in some cases that the superconducting state could not be maintained. Then, the inventors conducted a detailed investigation in order to clarify the cause of the disappearance of the superconducting state.

That is, a CGR temperature sensor was installed in a superconducting coil inside a superconducting magnet, and a low current was applied from outside to measure the temperature by resistance. As a result, it became clear that the temperature inside the superconducting magnet was not always constant, but changed in a very short cycle. The results of this investigation are shown in FIGS.

FIG. 5 shows the relationship between the CGR resistance value and the temperature of each of the two CGR temperature sensors attached for inspection to the superconducting coils inside the two superconducting magnets A and B provided with the mold therebetween. FIG.
Is the elapsed time and the C inside the superconducting magnet measured by these two CGR temperature sensors in the actual continuous casting operation.
This shows the relationship with the GR resistance value. Therefore, FIG.
If the resistance value obtained in is converted into a temperature according to FIG.
The internal temperature of the superconducting magnet can be determined.

As shown in FIG. 6, since the temperature change of the superconducting coil occurs at a very early stage, it is difficult to imagine that the temperature rise occurs due to normal heat conduction and heat radiation. Moreover, since the cryo (cooling device) of the superconducting magnet has a heat insulating material added to a metal having a thickness of several tens of millimeters, it is unlikely that an external temperature change directly increases the temperature of the superconducting coil. Then, when the thermocouple was attached to the superconducting magnet and the measurement was performed in more detail, the temperature sometimes rose on a part of the wall surface of the superconducting magnet, especially on the bottom surface, and the heat input to the superconducting magnet was It was newly found that not only the outside air but also the radiant heat had a great effect.

Normally, there is a nozzle in the gap between the rolls for jetting cooling water to cool the slab, from which cooling water is always jetted and a water curtain is formed, so that radiant heat is cut off. Was thought to be. However,
The above investigation revealed that radiant heat from the gap between the rolls could pass through the water curtain and water vapor curtain to some extent and reach the superconducting magnet.

Therefore, when a superconducting magnet is used as a means for applying a magnetic field to molten steel in a mold of a continuous casting machine, heat transfer from radiant heat from the slab or cooling water vapor sprayed on the slab is required. It is important to block radiant heat and the like. Examples of such blocking means include: 1) a case where a heat shield is provided outside the superconducting magnet and spaced apart from the magnet; and 2) a case where a part or all of the outer wall of the superconducting magnet is formed of a heat shield. There are two types, but it is only necessary to block radiant heat by either one.

FIG. 7 shows a case where a heat shield is provided outside the superconducting magnet. In the figure, reference numeral 14 denotes a superconductive magnet, and reference numeral 15 denotes a heat shield. In addition, in the example of FIG.
Although the case where the heat shield 15 is provided on the entire outer surface of the superconducting magnet 14 has been described, it is not always necessary to provide the heat shield 15 on the entire outer surface of the superconducting magnet 14, but only on the bottom surface where the influence of radiant heat or heat transfer is greatest. Just do it.

As the heat shield 15, any material having a heat insulating effect is suitable, and the material and structure thereof are not particularly limited. B) a structure at least whose surface is made of a substance having a high thermal reflectance, c) a structure that is cooled from inside or outside by a cooling fluid, and the like.

Next, FIG. 8 shows a case where the entire outer wall of the superconducting magnet is constituted by a heat shield. Reference numeral 16 in the figure denotes an outer wall formed by the heat shield. In this case as well, the material and structure of the heat shield are not particularly limited, but the structures a) to c) described above are advantageously adapted similarly to the above case. is there.

When the casting speed is reduced, such as at the start or end of continuous casting, or when the casting speed is changed during casting, it is desirable to control the electromagnetic force accordingly. However, when the current flowing through the superconducting coil is changed, a loss called an AC loss occurs in the superconducting conductor. Usually, AC loss is roughly classified into hysteresis loss generated in a superconducting filament such as NbTi and coupling loss generated in stabilized copper which is a base material for embedding the superconducting filament. Then, as the speed change of the magnetic field increases, any loss increases. As described above, AC loss does not occur when operating with a constant static magnetic field, but when the static magnetic field is changed (during excitation or demagnetization), the magnetic field applied to the superconducting conductor changes. Then, AC loss occurs and the temperature of the conductor rises.

Therefore, the inventors have tried various improvements on the structure of the superconducting magnet. For example, a method of immersing and cooling a superconducting coil directly in liquid helium was also studied. However, this method requires the provision of a helium cooling channel within the superconducting coil,
The cross-sectional area of the coil increases, and the required conductor length also increases. In addition, in the case of the immersion cooling method, a liquefaction machine is required, so a larger liquefaction / refrigeration device is required compared to the conduction cooling system, and the required number of refrigerators increases and the cooling device becomes huge, It becomes difficult to store the magnet in the confined space in the continuous casting machine. Also, by providing a cooling channel, the coil cannot be integrated with epoxy resin, so the conductor moves by electromagnetic force, and the temperature of the conductor rises due to frictional heating at that time, and the superconducting coil transitions to normal conduction (quenching) Problems are more likely to occur. Therefore, the above method is practically difficult to use in applications where the equipment space is severely restricted and the use conditions are severe, such as continuous casting equipment.

Therefore, the inventors have further devised and found that the above problem can be solved by using a conductor having a smaller NbTi filament diameter than the conventional one. FIG. 9 shows the results of a study on the relationship between the diameter of the superconducting wire and the demagnetization time when the diameter of the superconducting wire is variously changed when the conduction cooling system is used. Here, the demagnetization time is the time required to change the magnetic field from 1T to 0.3T. As shown in the figure, by reducing the superconducting wire diameter from the conventional 15 μm to 5 μm or less,
The degaussing time was significantly reduced. A particularly preferred wire diameter is 1 μm or less. It has been confirmed that the above-described relationship between the diameter of the superconducting wire and the demagnetizing time is similarly established in the case of the exciting time.

Therefore, according to the above-mentioned improvement measures, it is possible to effectively suppress the occurrence of AC loss and increase in the temperature of the conductor, so that there is no need to change the coil structure or the cooling structure.
Excitation / demagnetization time can be reduced.

[0043]

EXAMPLE As a continuous casting machine, a vertical bending type continuous casting machine shown in FIG. 10 was used. In the figure, reference numeral 17 denotes an immersion nozzle, 18 denotes molten steel, 19 denotes mold powder, and 20 denotes a roll. Also,
The continuous casting conditions are as follows.・ Nozzle: 2-hole nozzle (discharge hole diameter: 90 mm × 90 mm square) ・ Pouring speed: 3.6 m / min ・ Vertical section of vertical bending continuous casting machine: 2.5 m ・ Mold size: 1.50 m (width) × 0.20 m (thickness) -Applied magnetic field: 1T-Cast steel type: C: 300 to 350 ppm, Mn: 0.15 to 0.2 mass%, P: 0.025 m
ass% or less, S: 0.015 mass% or less, Al: 0.025 to 0.038 mass%, T.
O .: 25 to 35 ppm ・ Tundish molten steel temperature: 1550 to 1565 ° C. ・ Amount of casting: 300 ton (1 charge) Method A: The present invention (a method of vibrating a superconducting magnet in synchronization with a mold) Method B: Conventional method (Method of Fixing Superconducting Magnet and Vibrating Only Mold) Continuous casting was performed by both methods. Table 1 shows the results of examination of the stress applied to the mold and the superconducting coil during casting and the maximum internal temperature of the coil.

[0044]

[Table 1]

As shown in Table 1, according to the present invention, the load applied to the superconducting coil and the rise in temperature inside the coil, which have been problems in the past, can be effectively improved. It is also advantageous in increasing the negative strip rate and reducing the breakout occurrence rate.

[0046]

As described above, according to the present invention, oscillation (mold vibration) at a high frequency becomes possible, and as a result, high-quality continuous slabs can be produced by high-speed continuous casting.

[Brief description of the drawings]

FIG. 1 is a view showing the principle of generation of a mold reaction force.

FIG. 2 is a diagram showing a support point of a conventional superconducting coil.

FIG. 3 is a plan view (a), a front view (b) and a rear view (c) showing the internal structure of a superconducting coil according to the present invention.

FIG. 4 is a diagram showing a procedure for supporting a superconducting coil with three pull cords.

FIG. 5 is a diagram showing a relationship between resistance and temperature of a CGR temperature sensor attached to a magnet coil.

FIG. 6 is a diagram showing a relationship between an elapsed time in a continuous casting operation measured by two CGR temperature sensors and a CGR measurement value inside a magnet.

FIG. 7 is an explanatory view of a state where a heat shield is installed on a superconducting magnet.

FIG. 8 is an explanatory diagram in the case where the entire outer wall of the superconducting magnet is formed of a heat shield.

FIG. 9 is a graph showing the relationship between the wire diameter and the demagnetization time when the wire diameter of the superconducting wire (NbTi filament) is variously changed.

FIG. 10 is a schematic view of a continuous casting machine to which the present invention is applied.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Mold 2 Superconducting magnet coil 3 Magnetic field 4 Induction current 5 Mold reaction force 6 Support material 7 Coil case 8 Horizontal cable 9 Pulling cable 10 Vacuum container 11 Refrigerator 12 Metal plate 13 Heat insulation sheet 14 Superconducting magnet 15 Heat shield 16 External wall composed of thermal barrier 17 Dip nozzle 18 Molten steel 19 Mold powder 20 Roll

Claims (5)

[Claims] 1. A magnetic field generator for generating a static magnetic field in a direction opposite to a long side wall of a continuous casting mold, comprising: an annular air-core superconducting coil; a coil case for fixing the coil; A plurality of cords for locking the coil case at predetermined positions in the vacuum vessel, a refrigerator connected to the superconducting coil via a metal coupling body, and keeping the coil at a very low temperature by conduction cooling; A vacuum device for controlling the pressure in the vacuum vessel, wherein the coil case is supported from the back by horizontal ropes having a double cylindrical structure, and supported by at least three pulling ropes from above and below. A magnetic field generator for continuous casting of steel. 2. The magnetic field generator for continuous casting of steel according to claim 1, wherein the coil case is supported from above and below by at least three pull cords on the back side of the coil case. 3. The magnetic field generator for continuous casting of steel according to claim 1, wherein a heat shield for blocking radiant heat from the slab is provided outside the magnetic field generator. 4. The method according to claim 1, wherein the NbTi superconducting wire having a wire diameter of 5 μm or less is wound in a race track shape, and after winding, the coil is integrated by vacuum impregnation with epoxy resin. Characteristic magnetic field generator for continuous casting of steel. 5. When performing continuous casting of steel using a continuous casting mold provided with the magnetic field generator according to any one of claims 1 to 4, while synchronously oscillating the mold and the magnetic field generator, A continuous casting method for steel, wherein the flow of molten steel in a mold is controlled using a strong static magnetic field generated by the magnetic field generator.